[英]canvas animation javascript functions and global variables
请,有人可以帮我! 我是javascript新手。
我想使用javascript制作画布动画。 但是我有以下错误
SCRIPT5007:无法获取属性'getContext'的值:对象为null或未定义drawing_script.js,第31行字符5
这是代码。
使用Javascript:
// JScript source code
/*
Because the canvas can hold only one context at time, we'll create two canvas. Each one with its context.
One canvas for the circle, and another one for the square.
*/
var canvasCircle;
var contextCircle;
var x = 400;
var y = 300;
var dx = 2;
var WIDTH = 800;
var HEIGHT = 600;
// the circle wont make any transsformation.
function draw_circle(x, y, r) {
contextCircle.beginPath();
contextCircle.arc(x, y, r, 0, 2 * Math.PI, true);
contextCircle.closePath();
contextCircle.stroke();
}
function clear_canvas() {
contextCircle.clearRect(0, 0, WIDTH, HEIGHT);
}
function init() {
//canvasCircle = document.getElementById("canvas_circle");
canvasCircle = document.getElementById("canvas_circle");
contextCircle = canvasCircle.getContext('2d');
return setInterval(draw, 10);
}
function draw() {
// clear_canvas();
draw_circle(x, y, 50);
// if (x + dx > WIDTH || x + dx < 0)
// dx = -dx;
// x += dx;
}
init();
HTML5:
<!DOCTYPE HTML>
<html>
<head>
<meta charset="UTF-8" />
<script type="text/javascript" src="Scripts/drawing_script.js" language="javascript"></script>
<title>Blackberry</title>
</head>
<body>
<div class="drawing" style="background:Green">
<canvas id="canvas_circle" width="800" height="600"></canvas>
发生这种情况是因为您在创建画布之前执行了脚本。
创建画布元素FIRST,然后嵌入javascript。
IE:canvasCircle未定义,因为您无法通过尚不存在的ID获得元素!
我找到了答案: init()
应该是
function init() {
s_canvas = document.getElementById("canvas_square");
// Check if the canvas is supported and if the getContext is available
if (s_canvas && s_canvas.getContext) {
s_context = s_canvas.getContext("2d");
return setInterval(draw, 10);
}
else {
alert("Canvas is not supported!");
}
}
init()
的调用被window.onload=init
替换。
既然您说过您是javascript的新手,所以我认为问题可能在于您正在运行代码的浏览器可能不支持canvas。
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